Magnetic disk files provide primary data storage systems for computer systems. The data is recorded in concentric tracks of a magnetic disk in the form of magnetic transitions. The disks are mounted on a spindle and the information is accessed by an actuator which moves a magnetic transducer radially over the surface of the disk and aligns the transducer with the concentric tracks. The disk and spindle are mounted for rotation on a support shaft and the disks are rotated at high speeds by means of an electric motor.
Important requirements for magnetic disk files are quick access to data together with high data rates. A key to both is a high rotational speed. On average, it takes half a revolution of the disk for the desired data to reach the transducer after the actuator has positioned the transducer at the desired track. Thus, the higher the speed the disk rotates, the quicker the desired data can be accessed. Similarly, faster rotation of a disk causes more data to pass the transducer, increasing the data rate at the transducer.
Increased capacity is also important and has been accomplished by increasing both the data density per disk and the number of disks in a given space. The number of disks has been increased by packing the disks closer together.
The combination of higher spindle speeds and the increased number of disks has resulted in increasing the operating temperatures of high capacity, high performance disk drives. The increased heating comes from an increased torque requirement mostly due to the increased viscous dissipation of the disks, due to higher speed and the increased number of disks. The increased temperature has a compounding effect in that the increased temperature of the motor reduces the motor efficiency and increases resistivity, thereby increasing the temperature even further due to the increased winding resistive loss.
The increases in temperature lead to degraded spindle bearing reliability and reduced motor capability. The bearing reliability is reduced due to the increased chance of grease loss due to reduced viscosity. The motor capability is reduced not only because the torque constant is reduced when design speed is increased (due to the fixed voltage supply), but also because the increased temperatures lead to increased voltage drops across the resistive elements in the system, e.g., coil windings, transistors, etc. Specifically, the increased temperatures increase the resistance of the coil, so that there is increased voltage drop across the coil. Thus, the voltage available for voltage margin, or headroom, is reduced. In order to provide the voltage margin needed for speed control (headroom) an even lower motor torque constant may be required.
A solution to this problem is to reduce both the motor and bearing temperature by increasing the heat transfer out of the motor and to the base plate and cover where the heat can be removed by convective heat transfer. Spindle motor configurations which place the stator below or outside both bearings can greatly improve the heat sinking of motor heat losses into the base plate, but these designs compromise bearing span or bearing size constraints. Also, some of these designs do not allow support of the hub during a heat shrink installation of disk clamps. In addition, the motor volume in such designs may be compromised compared to the more traditional design with the stator between the bearings.
The traditional stator between the bearings design is optimal for efficiency and highest spindle pitch stiffness. However, a difficulty with this design with respect to heat transfer is that the shaft must typically be a martensitic steel which has a coefficient of thermal expansion (CTE) (compared to the CTE of the other parts of the spindle) such that the bearing preload can be adequately maintained through a range of temperatures. Although the stator is attached directly to the shaft allowing heat conduction through the shaft to the base plate, the conductivity of typical shaft steels is poor (about 25 (W/m)/K--watts per meter per degrees Kelvin).
A fiber reinforced metal has been disclosed for improving the heat conduction effect of a motor. In Japan patent application no. 61-151762, published Jan. 18, 1988, a motor shaft is designed using aluminum alloy reinforced with a SiC whisker. The whisker reinforcing is claimed to retain (or increase) strength while increasing heat conduction. However, although the shaft improves the heat conduction effect and maintains strength, the issue of matching coefficients of thermal expansion is not discussed. The use of whisker presents a material which is non-uniform in cross section and which therefore cannot match a coefficient of thermal expansion of a uniform material.